1. Field of the Invention
The present invention relates to a developing method and a method for forming fixed images used for plain paper copying machines, laser printers, plain paper facsimiles, etc. Particularly, it relates to a toner projection developing method using a magnetic encapsulated toner which can be fixed at a low temperature and a low nip pressure, and a method for forming fixed images utilizing such a developing method and a low-temperature, low-nip pressure fixing method.
2. Discussion of the Related Art
When fixed images are formed by using copying machines or laser beam printers, a developing system is an important part of the entire system. Various kinds of methods and materials to be used for the developing system have been studied. Among the methods thus studied, two-component developing methods and one-component developing methods have so far been employed, in which a magnetic brush method is mainly utilized.
The basic principle for a two component developing method using a developer comprising a magnetic carrier and a toner containing a coloring agent, and a supply toner was proposed in 1952 (U.S. Pat. No. 2,618,551). Also, a magnetic brush developing method was proposed in 1953 (U.S. Pat. No. 2,786,439), and this method contributes greatly to an image-reproducibility and a miniaturization of a developing device. However, these two-component developing methods have such problems that retainability of the image quality is poor, that deterioration of the developer takes place, that controlling of the toner concentration is necessary, that it is unsuitable for the miniaturization of the developing device used, and that toner scattering is likely to take place.
On the other hand, since the developers used in the one-component developing methods do not contain a carrier, it is unnecessary to provide a device for keeping the toner concentration level constant and to provide a stirring device for the developer, so that the developing devices for the one-component developing methods can be more compact than those for the two-component developing methods.
A method using a conductive magnetic toner (specific resistivity: not more than 10.sup.8 .OMEGA..cm) is conventionally proposed as a one-component developing method (U.S. Pat. No. 3,909,258). However, since it is difficult to transfer a toner having such a low resistivity to a plain paper by corona charging, a magnetic toner having a higher specific resistivity of not less than 10.sup.15 .OMEGA..cm has been used at present. However, since a carrier is not used in the one-component developing method, various proposals have been made to achieve stable charging. Such methods include three types mentioned below.
(1) A method of simultaneously charging at developing by utilizing a latent image electric field (U.S. Pat. No. 4,121,931, Japanese Patent Laid-Open Nos. 50-92137, 53-31136 and 54-134640); PA1 (2) A method of supplying electric charges inside of the developing device (Japanese Patent Laid-Open Nos. 53-4549, 53-47936 and 59-33908); and PA1 (3) A method of forming a thin toner layer on a sleeve while charging the toner (Japanese Patent Laid-Open No. 50-45639). PA1 (1) A developing method using an apparatus comprising a nonmagnetic sleeve involving a magnet roller therein, the nonmagnetic sleeve being arranged opposite to a photoconductor with a gap therebetween in a thickness of more than that of a layer of toners on the sleeve, and a blade arranged opposite to the nonmagnetic sleeve, the method comprising the steps of forming a uniform layer of toners on the nonmagnetic sleeve with a blade; rotating the photoconductor and the nonmagnetic sleeve in the same direction at the gap; and supplying an alternating electric current with a frequency of 10 to 500 Hz, so as to produce an alternating electric field in the developing gap, wherein the magnetic toner is a magnetic encapsulated toner, which can be fixed even at a nip pressure of 0.01 to 0.1 kg/cm without a supply of silicone oil onto a surface of a heat roller; and PA1 (2) A method for forming fixed images comprising the steps of charging a photoconductor; exposing the photoconductor to light to form an electrostatic latent image on the photoconductor; developing the electrostatic latent image to form a visible image by the above developing method; transferring the formed visible image to a recording medium; and fixing the transferred visible image onto the recording medium at a temperature of not more than 120.degree. C. and at a nip pressure of 0.01 to 4 kg/cm.
Among them, the method (3) of forming a thin toner layer on a sleeve while charging the toner has been practically used in the toner projection methods disclosed in Japanese Patent Laid-Open No. 41-9475 and U.S. Pat. Nos. 3,866,574 and 3,890,929 and in the FEED method disclosed in Japanese Patent Laid-Open No. 58-211712. Among them, the toner projection methods are known as the preferred developing methods for the reasons that the background contamination can be prevented, and that the line images or gradations can be most suitably obtained under the optimum alternating electric field conditions such as voltage and frequency. This is because the toner layer and the electrostatic latent image-bearing surface are in non-contact, so that the development is carried out in the gap produced therebetween, and also because the development is carried out by projecting a magnetic toner on a magnetic brush by means of applying an alternating electric field using alternating current for developing bias.
On the other hand, since the encapsulated toner has such properties that it has an easy function separation for the core material and the shell, it has been known to be suitable for various methods for forming fixed images. A developing method of the toner projection using a magnetic encapsulated toner is disclosed in Japanese Patent Laid-Open No. 61-59363. However, the magnetic encapsulated toner disclosed in the reference is such that fixing is carried out at a high nip pressure of, for instance, 5 to 10 kg/cm. Therefore, the fixing apparatus is made undesirably large by the reinforcement of the fixing roller and the fixing device system, thereby making it difficult to miniaturize the overall printing apparatus. Also, when fixing is carried out by a pressure fixing at such a high nip pressure, the fixing strength is comparatively weaker than that of the thermal fixing method, thereby causing wrinkling of paper sheets.
Incidentally, in order to achieve the miniaturization of the printing apparatus, it is also important to carry out a low-temperature fixing. In other words, when the fixing is carried out at a high temperature, a considerable distance has to be maintained between the fixing device and the photoconductor or the developing device. Also, the forced radiation device itself is large. Therefore, it is necessary to use a large overall printing apparatus. Also, when fixing is carried out at a high temperature, problems, such as noise generated by the forced radiation device, high cost required for materials used in the periphery of the fixing device, troubles such as curling and jamming of paper sheets, and difficulty in quick printing, take place.
Therefore, the use of a toner which can be fixed at a low temperature and a low nip pressure has been long desired. However, such an encapsulated toner suitable for a toner projection development has not been known.
Further, with the aim of solving the above problems, the present inventors have found an encapsulated toner whose shell comprises an amorphous polyester (Japanese Patent Application No. 4-259088) and a thermally dissociating encapsulated toner (Japanese Patent Laid-Open No. 4-212169). These encapsulated toners have excellent offset resistance and fixing ability even at a low temperature and a low nip pressure, and also they have an excellent blocking resistance. Thus, clear images free from background contamination can be stably formed for a large number of copying in a heat-and-pressure fixing method using a heat roller.
If such encapsulated toners are produced as magnetic toners and can be applied to the toner projection development, the developing device can be made compact as described above due to the one-component developing method, and the miniaturization of the fixing device can be achieved by utilizing a low-temperature, low-nip pressure fixing, thereby remarkably miniaturizing the overall apparatus. However, it has not been known whether these encapsulated toners for a low-temperature, low-nip pressure fixing are applicable for the toner projection development. Particularly, since the encapsulated toner in the present invention can be fixed even at a remarkably low nip pressure of, for instance, about 0.01 to 0.1 kg/cm, such problems as breaking of the shell of the encapsulated toner due to the projection impact and adhesion of the broken substances onto the surface of the photoconductor during the toner projection development are likely to take place. Thus, although there has been a great demand for miniaturizing the apparatus, it has been considered unsuitable or even difficult to apply these encapsulated toners which can be fixed at a remarkably low nip pressure to the projection development.